The present invention relates to a fusible link and more particularly to a fusible link which is not fused or melted by an overcurrent in a rare-short-circuit range.
Conventionally, fusible links (F/L) with an overcurrent of 200% or more of a conducted, or rated, current have been used as a protective fuse for an electric circuit such as a motor load circuit through which an overcurrent flows. For this purpose, fusible links with an overcurrent of 200% or more have been demanded to efficiently protect such an electric circuit when a burst current (a dead-short-circuit occurs) is generated. Namely, a current range is divided based on a boundary value. A current flowing at the boundary value is as large as twice a rated value. A dead-short-circuit range is a current range larger than the boundary value and a rare-short-circuit range is a current range smaller than the boundary value. A fuse having characteristics which are effective for both the dead-short-circuit range and the rare-short-circuit range is demanded.
When a large overcurrent flows as when a dead-short-circuit occurs, a fusible link needs to ensure the interruption of a load circuit before the circuit is damaged or a lead wire connected to the load circuit is fused or fumes. In addition, for example, when an electric powered window of a motor vehicle is opened or closed, although a motor lock current, in an intermediate current range, flows through a circuit with an overcurrent of 200% or less for on the order of about 10 seconds, the circuit needs to be prevented from being interrupted easily even though the motor lock current flows frequently.
As a fuse having characteristics like those described above, there has been proposed a fuse which includes a pair of connecting portions which are opposed to each other, and a fusible element portion (a fuse portion) which is provided at an intermediate portion of one of the connecting portions and fixes a metal chip by a surrounding attachment portion (refer to Patent Document 1, for example). Here, the metal chip is a linear material which is produced by extruding a low melting point metal into a linear shape and cutting it to a predetermined length, and the fusible element portion is made up of a plate-shaped fusible metallic conductor. A base material of the fusible element portion is an alloy of copper which constitutes a conductor wire, and the fusible element portion is made to be fused momentarily when a large current flows through a cross-sectional area which is smaller than an other portion. On the other hand, the material of the metal chip is tin (Sn) whose melting point is lower than that of copper (Cu), and when electrified, the metal chip heats and melts to be dispersed within the fusible element portion for formation of an alloy phase. Therefore, in the small current range to intermediate current range, the fusible element portion is fused in the alloy phase whose resistance is higher than that of the copper alloy which constitutes the base material thereof. In this way, the fuse having the low melting point metal such as tin or alloy containing tin as its main constituent changes its fusing time with respect to conducted current depending upon the mass of tin. Conventionally, the fuse of this type utilizes a solid metal chip, and the fusing characteristics thereof have been controlled by changing the dimensions of the solid metal chip utilized.
In the conventional fuse that has been described above, however, when an overcurrent in the rare-short-circuit range (for example, a small current range when overcurrent is on the order of 110%) flows, the dispersion of tin is progressed by Joule heat and the alloy phase is formed, whereby the fuse becomes easy to be fused or heated. Because of this, it becomes difficult for the fuse to be kept unmelted for a long period of time (for example, 10 hours or longer).
[Patent Document 1] JP-A-5-166453